Amber is a natural, amorphous, polymeric glass which is formed from the fossilized resin of plants. The plant resins from which amber originates comprise complex mixtures of terpenoid compounds, acids, alcohols, and essential oils. As the resin ages, it becomes harder and forms a semifossilized product known as copal. Amber and copal are distinguished by their physical characteristics, e.g., melting point, hardness, and solubility. In particular, amber has mechanical, dielectric, and thermal features common to synthetic polymeric glasses.
Natural inclusions in amber represent organic material such as pieces of leaves or small insects that were trapped in the plant resins before the liquid resin hardened. The chemical structure of the resin changes during fossilization, e.g., the linking of isoprene units such as diterpenoids causes inert dehydration of organic materials present in the amber.
Insects trapped in amber have been studied for many years because they can provide valuable clues to entomologists regarding the phylogeny of species. Recently, it has been discovered that "ancient" DNA can be recovered from samples of insects or plants trapped in amber, even though the estimated age of the species has been over many millions of years in some cases (Cano, Endeayor, 20, 162 (1996); DeSalle and Grimaldi, Current Op. Genet, & Devel., 4, 810 (1994); Poinar, Experieltia, 50, 536 (1994)). However, the ancient DNA in fossilized amber is often present at very low concentrations and may be quite degraded. While an amplification reaction, e.g., the polymerase chain reaction (PCR), can increase the concentration of DNA recovered from amber to detectable levels (Cano et al., U.S. Pat. No. 5,593,883), contamination during the DNA recovery process or subsequent analysis, and the presence of amplification inhibitors in the sample, e.g., tannins, porphyrins, heme and the like, can render PCR-derived results unreliable or unobtainable.
The integrity of ancient DNA which is embedded in amber may be compromised by oxidation of the DNA bases, a reaction that affects the ability of PCR to correctly amplify DNA sequences (Paabo et al., J. Biol. Chem., 265, 4718 (1990)). Oxidation reactions, however, are not directly involved in the breakage of the DNA backbone. DNA integrity may also be affected by depurination (hydrolysis of the deoxyribose/adenine or guanine bond), followed by a .beta.-elimination reaction that results in chain breakage. This reaction is thought to be the main reaction important in the fragmentation of DNA in the geologic environment. Nevertheless, based on studies of the retardation of the racemization of amino acids in insect tissues embedded in amber, Bada et al. (Geochim. Cosmo. Acta, 58,3131 (1994)) suggested that the breakdown of DNA embedded in amber might be inhibited.
Preservation of isolated and/or purified biological samples, e.g., isolated protein or nucleic acid, is often accomplished by storing the sample at low temperature, e.g., at -20.degree. C. or -70.degree. C. Low temperature slows natural biological and chemical processes, which can lead to the degradation of cellular components such as carbohydrates, proteins and nucleic acids. Moreover, the lower the temperature, the slower the degradation process. For example, complex cellular samples, e.g., sperm and eggs, are frozen and stored in a container having liquid nitrogen, which maintains the sample at about -200.degree. C. However, to preserve the sample, the level of liquid nitrogen in the container must be carefully monitored and the container must be periodically replenished with liquid nitrogen.
Thus, there is a need for an improved method to preserve isolated vertebrate nucleic acid.